We report the first detailed studies of the photochemistry of single molecules in aqueous solution. Photolysis of single B-phycoerythrin (B-PE) molecules, a highly fluorescent phycobiliprotein containing 34 bilin chromophores, is observed as an abrupt cessation of the fluorescence emission from individual molecules as they flow through a similar to 40 pL probe volume. These measurements demonstrate that B-PE is photolyzed in a single photochemical step. The observation of abrupt photobleaching of single molecules could not be made in a bulk measurement because the signal would be averaged over many molecules to yield a continuous decay. Photon pair correlation measurements of the fluorescence detected from single B-PE molecules demonstrate that the molecule behaves as a single quantum system, not as a collection of 34 independent chromophores.